Habitable Planets: A Splendid Isolation?

by Paul Gilster on September 24, 2007

Our assumptions about terrestrial planets seem pretty straightforward. We’re only now reaching the level where detecting such worlds becomes a possibility, with advances in ground- and space-based telescopes imminent that will begin to give us an idea how common such planets are. Hoping for the best, we assume Earth-sized worlds in relatively comfortable places are common and even extend our search from G and K-type stars to the much dimmer (and more numerous) M-dwarfs.

But what do we mean by a terrestrial planet? Size is an obvious criterion, but so is placement in the kind of habitable zone we would find conducive to our kind of life. That means liquid water at the surface. So far so good, but keep a sharp eye on the wild card in all this: Orbital ecccentricity. It’s a measure of how far the orbit of a planet deviates from a circle, and we need to know more about it. Obviously a highly eccentric orbit could swing a planet through the habitable zone and right back out again, never allowing a stable and benign environment for life to develop.

Many of the planets already discovered show fairly eccentric orbits. A short but intriguing paper by Daniel Malmberg (Lund Observatory, Sweden) and team now asks a provocative question: Is there a mechanism that ensures high values of orbital eccentricity, and if so, what does it tell us about planet formation in other solar systems? The assumption is that because most stars form in clusters, close encounters between young stars are fairly common. And that poses real problems.

For one thing, the orbits of planets in a given system could be profoundly altered by a close stellar pass, with some of them being ejected entirely. The planets remaining would then be left with significantly more eccentric orbits. A major question to ask is whether our Sun has ever had such a close encounter with another star. If not, that could explain the nearly circular orbits we see in our Solar System, and might also have something to do with the placement of the more massive planets far from the Sun, not the scenario in many of the exoplanetary systems we’ve examined.

These considerations mean that planetary systems that were once much like ours have been made into the kind of systems we have often observed, with planets on orbits so eccentric as to make the emergence of life problematic at best. Consider, for example, what can happen when a single star encounters not just one other star but a binary system:

If a single star instead encounters a binary system, it can be exchanged into it. When this occurs, the orientation of the orbital plane of the planets with respect to that of the companion star is completely random. This means that in about 70 per cent of the cases, the inclination between the two will be larger than 40◦. When that happens, the Kozai Mechanism will operate… Given that the binary is not too wide, the Kozai Mechanism will cause the eccentricities of the planets to oscillate. If the planetary system contains multiple planets, this eccentricity pumping can cause strong planet-planet interactions, causing the orbits of the planets to change signiﬁcantly and sometimes also ejecting one or more planets.

If so-called ‘singletons,’ formed singly and with no history of close stellar interactions, are the only places where Solar Systems like our own can form, we have placed a constraint on habitable terrestrial worlds. How much of one? We begin by ruling out a vast range of multiple star systems. As to solitary stars with Sun-like masses, the authors have numerically simulated a range of stellar clusters like those in which our Sun formed. They conclude that five to ten percent of all planetary systems around such stars have been altered by dynamical interactions as well, most likely to the detriment of life’s chances there.

A history of isolation, then, may play a role in the habitability of any terrestrial world around a Sun-like star. But note: The ‘if’ in the above paragraph is called into question by a good deal of recent work on the stability of planetary orbits in binary systems. The paper is Malmberg, Davies et al., “Is our Sun a Singleton?” to be published in the proceedings of IAUS246 “Dynamical Evolution of Dense Stellar Systems” (abstract).

I’m not convinced that an eccentric orbit would be a dealbreaker for habitability. We know that planetary atmospheres help cancel out temperature fluctuations to an extent; more heat produces more clouds that block sunlight. And we know that oceans are good at retaining heat; we used to think tide-locked worlds would be uninhabitable, but now we know their oceans and atmospheres could spread out their heat more evenly.

Sure, if a planet’s eccentric orbit brought it too close to its star, the volatiles would eventually boil away. But if its apastron were in the habitable zone and its periastron further out, then a fairly large planet with a substantial ocean could potentially remain habitable year-round, especially if it orbited a cool star and had a short year. It would have pretty extreme seasons, but life on Earth thrives in a wide range of climates.

Certainly it would reduce the chances of life developing. But it would not eliminate it. Trees can survive a hash winter frozen solid and yet in the spring they are unharmed. A desert can be dry for years, yet when there is water life will grasp the chance.

Life on such a world might find interesting ways to survive such changes. As the planet gets further from the sun and starts to cool the plant life might be able to alter its color so as to be nearly black – absorbing more the their sun’s heat. As the planet gets closer it might be able to make itself paler. Or, it might be that in addition to photosynthesis they would be able to use the sun light for other chemical reactions that produce heat. It would not help in the extreme cases but a mild to moderate eccentricity might not be beyond the adaptability of most biospheres.

It’s not a knock out punch to the chances of life arising, just a rather painful blow. And if life can arise on such a world then such a changing environment might actually promote the development of intelligence. Intelligence is the best mechanism this world has come across yet that can allow the larger animals to adapt to almost any environment, from the rain forest to the deserts to the arctic tundra.

Regarding eccentricity, this paper suggests even at quite high eccentricities, Earthlike planets can be stable.

As for whether it would “certainly reduce the chances of life developing”, I think it’s far too early to state something like that. If life begins in a deep-ocean, black-smoker environment, it would be quite well insulated against temperature changes.

The Darren Williams et. al. paper referenced above shows that oceans moderate temperature swings even for eccentricities as high as 0.4. For the e = 0.7 case Williams et. al. had to reduce the insolation levels to just 0.29, roughly Mars-like. The temperature swings were incredible, but some patches of the planet were still viable.

For a popularised version “Discover” magazine had an article a few years back “Circles of Life”…

But the shift in orbit would also have a chance of changing a planet’s orbit so that it spends a part of its time inside the orbit of mercury. (assuming a sol like system for discussion purposes) That I think would be a killer. As close as venus to as far as jupiter I could see some chance of life developing and surviving. But not mercury. So a certain percentage of planets so affected would not have the possibility of developing life.

On the other hand, maybe some worlds that would never have developed life before would be tossed into an orbit where it might be possible. What would happened to venus if its orbit were altered so that it passed outside of mars? What would happen to mars if its orbit were altered so that it swung inside of earths orbit.

One thing I don’t see in the above comments is the effect of axial tilt. For Earth, the seasonal variation in local insolation at mid and high latitudes due to axial tilt is far greater than the peak-to-trough 6-7% difference due to orbital eccentricity. That’s why we can have winter in the northern hemisphere around perihelion.

With higher orbital eccentricity and less axial tilt a planet could get similar seasonal variation to what we experience. Except the seasons would be synchronized globally, even at the equator. That would surely be compatible with life.

Williams also published papers on the effect of obliquity. That’s also how Hal Clement’s fictional planet Mesklin can have (seasonal) methane oceans during its summer, which is otherwise hot enough for liquid ammonia.

Problem is that obliquities aren’t very stable without an anchor mass, like our moon, thus a lot of promising configurations are only fleeting.

Yet that does make for an interesting scenario. Two nearby planets can trade high eccentricities back and forth on a regular basis over many orbits, thus creating a “Great Cycle” which moves a planet from essentially zero eccentricity to very high and back again. Life could conceivably evolve adaptations to such extremes and/or experience waves of population advance and contraction. Such worlds could have incredibly diverse biospheres and rates of evolution as remnant populations re-expanded and merged with their counterparts in the opposite hemisphere.

Adam, good points. We also need to keep in mind that orbit eccentricity is not constant. The long-term models run by Wisdom and others show a lot of variation in eccentricity of planets in our own solar system in periods as short as several million years. Axial tilt, eccentricity, stellar output, and perhaps other factors all vary and sometimes their cycles work together or at odds to cause interesting climate changes. For example, in one era there may be only one habitable hemisphere and in others both may be similar, just due to different length cycles of tilt, precession and eccentricity.

I suspect some relative stability is needed to get life started, then once evolution goes to work life can adapt to reasonable departures from the average where that average makes for habitability. Climate itself will shift due to life as it modifies the environment (e.g. oxygen atmosphere).

Abstract: We report the detection of two very eccentric planets orbiting HD4113 and HD156846 with the CORALIE Echelle spectrograph mounted on the 1.2-m Euler Swiss telescope at La Silla. The first planet, HD4113b, has minimum mass of $m\sin{i}=1.6\pm0.2 M_{\rm Jup}$, a period of $P=526.59\pm0.21$ days and an eccentricity of $e=0.903\pm0.02$. It orbits a metal rich G5V star at $a=1.28$ AU which displays an additional radial velocity drift of 28 m s$^{-1}$/yr observed during 8 years. The combination of the radial-velocity data and the non-detection of any main sequence stellar companion in our high contrast images taken at the VLT with NACO/SDI, characterizes the companion as a probable brown dwarf or as a faint white dwarf. The second planet, \object{HD 156846 b}, has minimum mass of $m\sin{i}=10.45\pm0.05$ M$_{\rm Jup}$, a period of $P=359.51\pm0.09 $ days, an eccentricity of $e=0.847\pm0.002$ and is located at $a=1.0$ AU from its parent star. HD156846 is a metal rich G0 dwarf and is also the primary of a wide binary system ($a greater than 250$ AU, $P greater than 4000$ years). Its stellar companion, \object{IDS 17147-1914 B}, is a M4 dwarf. The very high eccentricities of both planets can be explained by Kozai oscillations induced by the presence of a third object.

Abstract: This review focuses on recent results in advancing our understanding of the location and distribution of habitable exo-Earth environments. We first review the qualities that define a habitable planet/moon environment. We extend these concepts to potentially habitable environments in our own Solar System and the current and future searches for biomarkers there, focusing on the primary targets for future exploratory missions: Mars, Europa, and Enceladus. We examine our current knowledge on the types of planetary systems amenable to the formation of habitable planets, and review the current state of searches for extra-solar habitable planets as well as expected future improvements in sensitivity and preparations for the remote detection of the signatures of life outside our Solar System.

Abstract: About half of all known stellar systems with Sun-like stars consist of two or more stars, significantly affecting the orbital stability of any planet in these systems.

Here we study the onset of instability for an Earth-type planet that is part of a binary system. Our investigation makes use of previous analytical work allowing to describe the permissible region of planetary motion. This allows us to establish a criterion for the orbital stability of planets that may be useful in the context of future observational and theoretical studies.

The search for life beyond the Earth is closely linked with hunting for habitable worlds. Astronomers have always hoped to find planets in the so-called “Goldilocks zone” around their parent stars, where the temperature is just right.

Liquid water is a key ingredient for life as we know it, and this is one reason why the Earth is in an ideal location. Any closer to the Sun and water would boil away into space; any further out and it would freeze.

This restricts our search as it limits the places we think life could exist, but new research hints that habitable zones could extend much farther than previously thought.

Siegfried Franck and a team at the Potsdam Institute for Climate Impact Research and from Warsaw University have been studying frozen extrasolar planets.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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